1. Field of the Invention
The present invention generally relates to an image display apparatus, and more particularly, to a projection-type image display apparatus including an optical modulator which is illuminated by light from an illuminator, and a projection lens which forms the modulated light from the optical modulator into an image.
2. Description of the Related Art
A typical one of the conventional projection-type image display apparatuses includes an illuminator, optical modulator (will be referred to simply as “modulator” hereunder) illuminated by light from the illuminator, and a projection lens which forms the modulated light from the modulator into an image. The conventional image display apparatus uses a discharge lamp as the light source of the illuminator, and a liquid crystal as the modulator, and has a relatively large size.
Since the image display apparatus of the above type adopts a color filter for each pixel of one modulator and a so-called sequential color display method by which an image is displayed in colors on the time-shared basis, so it is inexpensive. However, the apparatus is not advantageous in that it cannot utilized light efficiently and consumes much power.
The reason why the apparatus cannot utilize the light with a high efficiency is that since the modulator is a non-luminous element which modulates the polarized state of incident light, a means is needed which splits a light beam emitted from a light source into rays of light according to the polarized state of each ray and then recombines the split rays of light together, the light source emits light even when an image is displayed in black, which is different from a luminous-type image display apparatus, and the apparatus loses light correspondingly to an light-utilization efficiency which depends upon the numerical aperture of the modulator.
For a higher efficiency of light utilization in the conventional image display apparatus, the following are done by the optical elements etc. included in the image display apparatus.
[Splitting and Recombination of Polarized Light Components]
Referring now to FIG. 1, a conventional image display apparatus is schematically illustrated in the form of an axial-sectional view. The above-mentioned means for splitting a light beam emitted from a light source in an illuminator into rays of light according to the polarized state of each ray and then recombining the split rays of light together is known as a polarized-state converter. As shown, the illuminator used in the image display apparatus includes a light source 101, modulator 102, and a P-S converter 103 provided, as the polarized- state converter, between the light source 101 and modulator 102. The P-S converter 103 is formed as shown in FIG. 2. Namely, a glass block 108 is prepared by attaching glass plates 105 each having formed thereon a polarized component splitting layer 104 formed from a multilayer film of an inorganic substance and glass plates 107 each having a reflecting surface 106 formed thereon alternately to each other. The glass block 108 thus formed is sliced along cutting planes 109 laid obliquely in relation to the joined surfaces of the glass plates 105 and 107 to make P-S converter plates.
A light beam being a mixture of P- and S-polarized components, projected onto the P-S converter 103, is separated by the polarized component splitting layer into P- and S-polarized components. So, the P- and S-polarized components split by each of the layers of the P-S converter 103 will go out of the P-S converter 103. With a half-wave (λ/2) plate 110 provided at a portion of the P-S converter 103 at which the light will go out and corresponding to either the S- or P-polarized ray of light, the P-S converter 103 provides a light beam including solely either the P- or S-polarized component.
Use of the P-S converter 103 and half-wave plate 110 as a polarized component splitter permits to improve the efficiency of light utilization of the illuminator used to illuminate the modulator which modulates polarized components of an incident light.
In the above illuminator, a light beam emitted from the light source 101 is reflected by a parabolic mirror 111 and incident upon the P-S converter 103 through a pair of fly-eye lenses 112 and 113. Then, it passes through the half-wave (λ/2) 110 and a condenser lens 114 and reaches the modulator 102.
[Reflecting Polarizer]
The conventional polarizer allows only one of two types of polarized components of an incident light to pass through while absorbing the other type of polarized component. However, there has been proposed a “reflecting polarizer” which allows one of two types of polarized components of an incident light to pass through while reflecting, not absorbing, the other type of polarized component. Use of such a “reflecting polarizer” as the polarized-state converter permits to utilize the other type of polarized component of the incident light by reflecting it again, that is, to improve the efficiency of light utilization. [Linear Polarizer Using a Birefringent Multilayer Film]
Also, there has been proposed a linear polarizer using a birefringent multilayer film formed by laminating two types of polymer films each being anisotropic in refractive index and different in refractive index from each other and elongating the laminated films. In the linear polarizer, the laminated two types of polymer films are completely coincident in refractive index with each other in the direction of one of axes of polarization while being not coincident in the direction of the other polarization axis. By adjusting the different refractive indexes, it is possible to pass the polarized light in the direction of one of the polarization axes while reflecting the polarized light in the direction of the other polarization axis perpendicular to the one polarization axis. Thus, the “reflecting polarizer” can be provided.
Note that the above “reflecting polarizer” is commercially available from the 3M under the trade name “DBEF” or “HMF”.
[Circular Polarizer Using a Cholestetric Liquid Crystal]
As well known, the cholesteric liquid crystal selectively reflects light beams. There has been proposed a circular polarizer utilizing the property of the cholesteric liquid crystal to selectively reflect light beams. As disclosed in the Japanese Published Unexamined Application No. 281814 of 1994, since the cholesteric liquid crystal has a pitch varying by more than 100 nm, it can selectively reflect light beams having wavelengths over the visible range. Also, a wavelength-independent circular polarizer can be produced using such a cholesteric liquid crystal polymer-made circular polarizer.
The circular polarizer formed from the cholesteric liquid crystal polymer and polarized-state converter using the circular polarizer are known from he disclosure in the Japanese Patent Gazette No. 2,509,372. The invention disclosed in this Gazette utilizes the fact that because of the characteristic of the circularly polarized light, namely, since the phase changes 180 deg. per reflection, a right-hand circularly polarized light is converted to a left-hand one while a left-hand circularly polarized light is converted to a right-hand one. As shown in FIG. 3, the reflector or parabolic mirror 111 can be combined with a cholesteric liquid crystal polymer layer 115 to build a polarized component splitting/recombination unit. The polarized component splitting/recombination unit using the above linear polarization needs a half-wave (λ/2) plate, but the polarized component splitting/recombination unit using the circular polarization needs no such half-wave plate.
More specifically, a light beam emitted from the light source 101 is incident upon the cholesteric liquid crystal polymer layer 115 through a condenser lens 116, while being reflected by the reflector or parabolic mirror 111 and incident upon the cholesteric liquid crystal polymer layer 115 through the condenser lens 116. At the cholesteric liquid crystal polymer layer 115, the circularly polarized light in one direction will be allowed to pass through while the circularly polarized light in the other direction will be reflected. The circularly polarized light in the other direction, thus reflected by the cholesteric liquid crystal polymer layer 115, is reflected by the reflector or parabolic mirror 111 to a circularly polarized light in the one direction, incident again upon the cholesteric liquid crystal polymer layer 115 and passed through the latter.
However, the conventional image display apparatus including the aforementioned illuminator is not advantageous in the following respects:
[Manufacturing Process]
The aforementioned illuminator can only be manufactured in a complicated process and with high costs.
[Problems of the Circular Polarizer Using the Cholesteric Liquid Crystal Polymer]
The circular polarizer disclosed in the aforementioned Japanese Published Unexamined Application No. 281814 of 1994 are independent of any light wavelength, but cannot be said to satisfactorily split an incident light into polarized components.
Therefore, to provide an image having a required contrast, the illuminator has to be used in combination with an absorbing polarizer (where one of the polarized components is allowed to pass through while the other polarized component is absorbed). Thus, it is difficult to utilize light with an improved efficiency.
[Problems of the Polarized Component Separator Using the Cholesteric Liquid Crystal Polymer-Made Circular Polarizer]
The illuminators shown in FIG. 3, disclosed in the aforementioned Japanese Patent Gazette No. 2,509,372 and Japanese Published Unexamined Application No. 281814 of 1994, respectively, are not always effective as expected when it is built in the form of a combination of a discharge lamp used as a light source in practice and a reflector or when it is used to illuminate the modulator.
More specifically, as shown in FIG. 4, the reflector 111 used with the discharge lamp has the actual sectional form of a paraboloid of revolution or an ellipsoid of revolution, and so a light beam reflected by a polarized component splitter formed from the cholesteric liquid crystal polymer layer 115 towards the light source 101 will be reflected twice by the reflector 111. On the assumption that the light beam has a phase change of 180 deg. when it is reflected once by the reflector 111, the phase change of 180 deg. given to the light beam reflected once will be canceled, namely, the light beam once reflected will have no phase change, when it is reflected once again.
Further, since the P- and S-polarized components of light are reflected with one reflectance and another, respectively, by the reflector 111 and they are changed in phase and scattered when they pass through the glass tube of the discharge lamp as the light source 101, so the polarized-state conversion will be less effective. Also, when the reflector is a parabolic mirror, a light beam emitted from the focal point will return to that point after it is reflected by a reflecting polarizer, but a light beam emitted from other than the focal point will not always return to that point after it is reflected by the reflecting polarizer.
Also, as shown in FIG. 5, when a spheroidal mirror is used as the reflector 111, the reflected light from the cholesteric liquid crystal polymer layer 115 will not return to the point of light emission of the light source 101 but will be absorbed by the electrodes of the discharge lamp and have the angular distribution thereof spread after it is reflected by the reflector 111, which depends upon the position of the cholesteric liquid crystal polymer layer 115. The spread angular distribution will increase the Etendue of the light source, which will cause the efficiency of light utilization to be lower.
As above, the polarized-state converter in the conventional illuminator is disadvantageous in efficiency of light utilization and manufacturing cost, and cannot return light to the light source with any adequate efficiency.